Nowadays, semiconductor devices such as semiconductor integrated circuits are used in a variety of fields, for example, for civilian use such as mobile communications terminals, display equipment, and laptop computers, as well as for industrial use such as various productive equipment and industrial robots. Meanwhile, with the expansion of application filed, a required reliability level is increased. Particularly, in the application fields involving human lives such as medical devices and transportation equipment, an extra-high reliability is required.
However, a semiconductor device having a general structure in which the device is formed on a semiconductor substrate is very vulnerable to ESD caused by, for example, a contact with a person retaining electricity, along with a refinement of rules of wiring on substrate. Thus, to protect such semiconductor device from ESD, an ESD protective element is provided in the input circuit of the semiconductor device.
FIGS. 28 and 29 are circuit diagrams showing the periphery of a protective element of an input circuit of a conventional semiconductor device. In FIG. 28, diodes 30 as ESD protective element are connected between an input terminal 13 and an upper voltage source terminal 15/a lower voltage source terminal 14 to protect each element of the input circuit using breakdown of p-n junction. In FIG. 29, an ESD protective element 31 is connected between an input terminal 13 and a lower voltage source terminal 14. The ESD protective element 31 is composed of several circuit elements, whose examples are disclosed in Patent Documents 1 to 3. The ESD protective elements described in Patent Documents 1 and 2 use breakdown of p-n junction of a transistor therein. The ESD protective element described in Patent Document 3 makes a zener diode and a field effect transistor therein operate with respect to application of overvoltage due to ESD and function as a protective element.
Since these conventional ESD protective elements are provided specially to protect circuit elements such as a transistor in the input circuit, the region for forming the ESD protective element is required in addition to the region which is needed in order that a semiconductor device has its original functions. In the production process, the step for forming the ESD protective element is also required.